Lets not go on money/staff, because if we did, Fox would win, and they're hardly reliable. I think in the case of the guardian, sure, it's their job to sell newspapers, but their sales method isn't lies and shocking headlines. The people who want that read the Sun. They base their marketing strategy on truthfulness - shocking headlines filled with lies could easily decrease their readership, not increase it, so all newspapers can't be grouped together.
The BBC on the other hand, who are they working for, directly? Not the taxpayer. The government. Sure, they have to remain sounding fairly unbiased, but slipping out the occasional lie among a load of truth is much more effective at working on the more intelligent segment of the population. The BBC could just totally ignore the government, and be totally unbiased, but, while the government couldn't directly punish them for it, there are always ways for them to be punished - spending cuts aimed towards them, not giving them coverage to certain events...
The BBC have a monopoly on the broadcasting in this country, and it seems to go largely unnoticed.